Consistently use the SOLISTENING() macro

Some code was using it already, but in many places we were testing
SO_ACCEPTCONN directly. As a small step towards fixing some bugs
involving synchronization with listen(2), make the kernel consistently
use SOLISTENING(). No functional change intended.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit f4bb1869ddd2bca89b6b6bfaf4d866efdd9243cf)